Cut three diagonal slits in both side of each fish. Place the garlic slivers into the slits. Place the fish in a single layer in a shallow dish. Whisk the oil, vinegar and rosemary, with ground black pepper to taste.

Pour over the fish, cover with clear film and leave to marinate in a cool place for about 1 hour. Put the fish on the rack of a grill pan and grill for 5-6 minutes on each side, turning once and brushing with the marinade. Serve hot, sprinkled with coarse sea salt and garnish with fresh rosemary sprigs and lemon wedges.